ATT (ACK)
ACK-based communication protocols
TCP is one of the communications protocols that relies on ACK signals to ensure successful data transmissions. When TCP packets are sent over a network, each packet contains an ACK number or flag, which is set to 1 in the packet header. This number indicates the sequence number of the next packet in the data stream that the destination station (device or process) should expect to receive.
So, in TCP, the destination acknowledges the received packets by sending back a packet containing the ACK bit set. A feature of TCP enables ACK to acknowledge that a series of TCP packets has been received instead of one packet. This enables many bytes of data to remain in flight, while minimizing delays. Like TCP, the ZMODEM protocol is also acknowledgement-based, meaning that it involves destinations positively acknowledging the receipt of messages sent by sources by sending ACK codes.
https://www.techtarget.com/searchnetworking/definition/ACK